The computational power of the brain depends on precise synaptic connections that link together billions of neurons. How changes in synaptic function mediate long lasting neuronal plasticity is a key question in modern neurobiology and likely to be important for understanding the pathogenesis associated with human cognitive disorders. Calcium influx into both pre- and post-synaptic neuronal compartments is a key determinant of synaptic transmission and long-term synapse potentiation. Presynaptically, calcium influx following an action potential initiates synaptic vesicle fusion and the release of neurotransmitters. Postsynaptically, calcium influx through neurotransmitter receptors triggers synapse-specific potentiation. However, the molecular mechanisms that allow calcium influx to mediate synaptic information transfer are still being elucidated. The goal of this project is to characterize the function of the synaptotagmin family in calcium-dependent membrane trafficking on both sides of the synapse. Synaptotagmin 1 (Syt 1) is a calcium-binding synaptic vesicle protein that functions as the fast calcium sensor for synchronous synaptic vesicle fusion. Although the basic functions of Syt 1 have now been identified, the mechanisms that underlie its activity are still unclear. Synaptotagmin 4 (Syt 4) is an evolutionary conserved member of the family that localizes to the postsynaptic compartment and is hypothesized to function in calcium-dependent postsynaptic vesicle fusion and the release of retrograde signals that mediate synaptic growth and plasticity. Characterizing Syt 4-dependent retrograde signaling will greatly facilitate our understanding of the role of postsynaptic vesicle fusion in neuronal plasticity. Our analysis will combine genetic manipulations available in Drosophila with molecular, biochemical, electrophysiological and morphological approaches to characterize how synaptotagmins mediate calcium-dependent membrane trafficking at synapses. [unreadable] [unreadable]